## Problem
The neogit commit (3d640c2) switched line background extmarks from
`hl_group`+`hl_eol` to `line_hl_group`. Due to [neovim#31151][1],
`line_hl_group` bg overrides `hl_group` bg regardless of extmark
priority. This made `DiffsAddText`/`DiffsDeleteText` intra-line
highlights invisible beneath line backgrounds — the extmarks were
placed correctly but Neovim rendered the line bg on top.
[1]: https://github.com/neovim/neovim/issues/31151
## Solution
Revert line backgrounds to `hl_group`+`hl_eol` where priority stacking
works correctly. Keep `number_hl_group` in a separate point extmark to
prevent gutter color bleeding to adjacent lines. The Neogit highlight
override (clearing their groups to `{}`) is independent and unaffected.
028ba5314e